Posted by developer:
Fixed in 8.0.21.
For debug builds, the server could exit trying to print an optimizer
trace.

[29 Jan 15:16]
Tor Didriksen

Posted by developer:
We decided not to use the contributed patch, but rather fix trace_tmp_table():
+ if (tab != nullptr && tab->join() != nullptr)
+ trace_tmp.add("in_plan_at_position", tab->idx());
The QEP_tab which had no index belonged to a QEP_TAB_standalone
which is documented as "not connected to any JOIN_TAB"